1. Field of the Invention
This invention relates to an absolute positioning servo system for positioning a shaft in response to a digital command input signal and more particularly to an absolute positioning servo system utilizing a resolver instrumenting the shaft to be positioned and a tracking resolver to digital angle encoder for providing a digital representation of the shaft position.
2. Description of the Prior Art
Prior art absolute positioning servo systems utilize absolute digital encoders coupled directly to the shaft to be positioned. The conventional absolute digital encoder technique utilizes multiple gear discs in an arrangement where each disc is read by a light source and a phototransistor arrangement. Experience has shown in many industrial applications that vibration and handling of absolute digital encoders has caused substantial maintenance and down time.
Another disadvantage with absolute positioning digital encoders is that the continuous indication of the shaft position is not available. That is, the shaft digital encoder must necessarily break the shaft position down into a number of discrete intervals or steps and the position of the shaft between these steps is indeterminable. Thus over the travel or position represented by the least significant bit (LSB) of the digital count, it is impossible to ascertain or control the position of the shaft with prior art servo control systems. For example, in a prior art servo control system to achieve an accuracy of 0.36.degree. the servo system must be designed so that full torque is available for a position error of 0.36.degree. or larger. Therefore, when the system reaches its null value, that is within a range of 0.36.degree., which contains the desired position, the digital encoder cannot provide any feedback due to the quantization of the feedback signal over the least significant bit. The prior art systems utilizing absolute digital encoders will therefore oscillate between 0 and 1 bit of position error and cause vibration in the equipment. This can cause equipment failures or malfunction and is undesirable.
In another prior art servo control system a digital angle is fed to one input of a translator while the sine and cosine functions of the instrumenting resolver are fed to the other input of the translator and an error signal representing the difference between the digital and analog inputs is provided out of translator. The error signal can be utilized to feed a servo system which drives a motor to position the shaft so that the error signals are minimum. This type of control system, while effective for very accurately positioning a shaft, does not give an indication of the shaft position at any instant of time. That is, the shaft will move the desired position as represented by the digital input, however, it is not possible to tell where the shaft is before it has reached its steady state position. This type of absolute positioning system does not provide an indication of the present shaft position. This signal is extremely important in systems where the instantaneous shaft position is required for display or position read out to other equipment.
Whenever the term resolver is used herein, it is understood to mean, resolver, synchro, differential transformer, control transformer or any other sinusoidal position indicating device. The output of resolvers are normally in suppressed carrier form, but it is customary to discuss the output as representing the sine and cosine of selected analog angles. Whenever sine and cosine functions are discussed herein, it is to be understood that these can represent signals in absolute or suppressed carrier form.